This is a great activity for modelling the digestive system that the students will love.

You will need per group (2-3 students):

• Potato masher or pestle
• The leg of a pair of tights (open at both ends)
• Ziploc bag
• 3 plastic bowls
• 2 sponges
• Vegetable oil
• Washing-up liquid in a bottle labeled ‘bile’
• Cereal with milk, bread, biscuits (any leftover food will do)
• Dilute hydrochloric acid in a beaker labeled ‘stomach acid’
• Paper towels
• Plastic gloves
• Universal indicator solution
• Five boiling tubes containing the following:
  • Water + blue food colouring (labeled ‘salivary amylase)
  • Water + red food colouring (labeled ‘pepsin’)
  • Water + yellow food colouring (labeled ‘trypsin’)
  • Water + green food colouring (labeled ‘pancreatic amylase’)
  • Water + pink food colouring (labeled ‘lipase’)

Health and safety: Always check for food allergies before you start. Students should wear plastic gloves and goggles when adding the stomach acid.

Lead-in

Even at Key Stage 3 most students have a good knowledge of the different parts of the digestive system so I usually begin with a simple labeling activity and ask one member of the class to record keywords on the board. I emphasise that the digestive system is essentially a long tube running from the mouth to the anus. I have a long piece of rubber tubing which is approximately the length of the alimentary canal (9 metres) which I show the students.

The mouth

I ask the students to put the food into one of the plastic bowls (the mouth). Large pieces of food need to be cut up into smaller bits (by the incisors) and then ground-up using the potato masher or pestle (the molars).

Highlight that this process of physically breaking down the food is mechanical digestion and that it greatly increases the surface area for chemical digestion by enzymes. Add salivary amylase (e.g. the water with blue food colouring) to begin starch digestion.

The esophagus

Tip the slop from the plastic bowl into the top of the tights and ask the students to squeeze the tights in order to push the bolus down the esophagus (this is modelling peristalsis) into the stomach (e.g. the Ziploc bag).

The stomach

Once in the stomach, add the stomach acid (students should test the pH first by adding universal indicator solution) and the pepsin. Seal the bag and churn it to mimic the mechanical digestion of the stomach. Keep going! The food can remain in the stomach for a long time.

Small intestine

Tip the contents of the stomach (the chyme) into the second basin (e.g the small intestine). Add a little vegetable oil to represent oils and fats in the food. Explain that the bile helps emulsify these lipids and neutralises the stomach acid to provide optimum conditions for the pancreatic enzymes. Add some washing-up liquid and give the basin a little shake – the oil should emulsify and settle on top of the liquid as tiny droplets. Add the trypsin, the pancreatic amylase, and the lipase.

Use the sponges to absorb some of the liquid. This models the absorption of nutrients into the blood stream.

Large intestine

Transfer the undigested food into the final basin (e.g. the large intestine) and start absorbing water using the paper towels. Finally, ask the students to model what is left into a stool which will then exit the digestive system via the anus (egestion).

Assessing learning

To follow up the activity and check the students’ understanding of the processes involved, I ask them to illustrate the ‘journey of a cheese sandwich’ as it passes through the alimentary canal. A fun way of doing this is to use large pieces of paper (3 or 4 sheets of A3 stuck together will do) onto which the students first draw their outline, then the parts of the digestive system.

Once complete, add some string so that the paper can be worn around the student’s neck to illustrate the passage of food in-situ. Disposable plastic aprons could also be used.